1. Field of the Invention
The present invention relates to a pellicle for lithography, in particular, a pellicle for lithography used as a dust guard of a mask for lithography when producing a semiconductor device such as LSI and VLSI or a liquid crystal display panel.
2. Description of the Related Art
In the production of a semiconductor device such as LSI and VLSI or a liquid crystal display panel, a pattern is formed by exposing a semiconductor wafer or an original plate for liquid crystal display panel. In this case, if dust particles adhere to an original plate for exposure (a mask for lithography) to be used, the dust particles absorb or bend light. Thus, a transferred pattern may be deformed, or an edge may be rough, and also the ground may be soiled to be black, resulting in degradation of dimensions, quality and an appearance or the like, which may lead to a problem of lowering performance or a production yield of a semiconductor device, a liquid crystal display panel, or the like.
For this reason, these processes are usually performed in a clean room. However, since it is difficult to always keep an original plate for exposure clean even in the clean room, there is adopted a method of attaching a pellicle as a dust guard which well transmits light for exposure to a surface of the original plate for exposure. In this case, dust particles do not adhere directly to the surface of the original plate for exposure, but adhere to a pellicle film. Accordingly, if a focus is set on the pattern of the original plate for exposure at the time of lithography, dust particles on the pellicle film do not affect the transfer.
An example of such a pellicle is shown in FIG. 2. In a pellicle 1, a pellicle film 3 is adhered to an upper end face of a pellicle frame 2 via generally an adhesive layer. Further a vent 7 can be provided at least on one side of the pellicle frame, and a dustproof filter 8 can be provided on the vent 7.
Such a pellicle is manufactured by adhering a transparent pellicle film made of nitrocellulose, cellulose acetate, or the like which well transmits light used for exposure, to a pellicle frame made of aluminum alloy such as A7075 subjected to black alumite treatment, stainless steel, polyethylene or the like, via a good solvent for the pellicle film applied to the upper side of the frame and air-dried (for example, see Japanese Patent Laid-open (Kokai) No. 58-219023), or via an adhesive such as acrylic resin and epoxy resin (for example, see U.S. Pat. No. 4,861,402 specification or Japanese Patent Publication (Kokoku) No. 63-27707). Furthermore, in order to attach an original plate for exposure, a sticking layer made of polybutene resin, polyvinyl acetate resin, acrylic resin, or the like, and a sticking layer protection liner for protecting the sticking layer can be provided on the lower part of pellicle frame.
In recent years, as a semiconductor device has been further miniaturized, a pattern rule in LSI or the like has become finer up to sub-quarter micron. Along with that, wavelength of a light source for exposure has become shorter. Namely, it has become shorter from g-line (436 nm) and i-line (365 nm) which had been mainly used, to a KrF excimer laser (248 nm) and an ArF excimer laser (193 nm). And recently, an F2 laser (157 nm) which generates light of further shorter wavelength is to be put into practice as a light source for exposure. In particular, in the case of using the F2 laser as a light source for exposure, there has been a problem in practicability of the pellicle in which an amorphous fluoropolymer is used as a pellicle film. This is because, even if an amorphous fluoropolymer which has been used for conventional lithography with the KrF or ArF excimer laser is utilized, it has low transmittance in the F2 laser beam.
Then, silica (silica glass) has come into notice as a material for a pellicle film with high transmittance and high light stability against an F2 laser beam. However, even silica absorbs light of 157 nm to some degree, and practical transmittance for a pellicle can not be obtained. In recent years, it was found that fluorine-doped silica has high transmittance against light of 157 nm in wavelength. Therefore it has been proposed that it should be used as a material of a pellicle film for a pellicle used for an F2 laser (for example, see Japanese Patent Laid-open (Kokai) No. 2000-292908).
Fluorine-doped silica has high transmittance and high light stability against light of 157 nm. However, in order to prevent flexure due to its own weight when it is adhered to a pellicle frame as a pellicle film, it needs more than 800 μm in thickness. In the case of thickening the pellicle film as described above, from an optical point of view, it is impossible to ignore the pellicle film in a lithography process. Thus, this must be taken into consideration when designing an aligner. Therefore, extremely high flatness, low flexure and low gradient are required, and it is very difficult to attach the pellicle to a reticle (an original plate for exposure).
In the case of utilizing a pellicle frame made of aluminum alloy as general when fluorine-doped silica is used as a material for a pellicle film, it is difficult to obtain mechanical flatness, and there is also difficulty in attachment. Moreover, it is difficult to use the pellicle frame made of aluminum alloy in practice since it deforms the fluorine-doped pellicle film depending on change of temperature.
As a result, it is not avoidable to use as a pellicle frame a silica glass, of which thermal expansion coefficient is the same as that of fluorine-doped silica. However, if the silica glass is actually used as a pellicle frame, it should be too expensive, and thus, the problem in cost occurs.
On the other hand, if the pellicle film made of fluorine-doped silica that is processed to be extremely thin, for example, to be several tens of micrometer in thickness, is used for a pellicle, the optical problem is overcome. That is, it does not need to take this into consideration when designing an aligner. However, it is difficult to produce such a thin pellicle film made of fluorine-doped silica through, for example, polishing. Even if it can be processed through polishing or the like, it is easy to break. Therefore, it possibly causes damage to an exposure system since it breaks soon. Consequently, it has not been of practical use.